A liquid crystal display device is being widely used for monitors of personal computer or portable device and for televisions because of its various advantageous features, for example, operation with low voltage/low power-consumption and reduction in size/thickness of film can be realized. For such a liquid crystal display device, various modes have been proposed according to the aligned state of liquid crystal molecules in the liquid crystal cell. Conventionally, a TN mode having an aligned state such that liquid crystal molecules are twisted at about 90° toward the upper substrate from the lower substrate of the liquid cell is predominating.
In general, a liquid crystal display device comprises a liquid crystal cell, an optical compensatory sheet and a polarizer. The optical compensatory sheet is used for canceling the coloration of image or enlarging the viewing angle, and a film obtained by coating a liquid crystal on a stretched birefringent film or transparent film is used therefor. For example, Japanese Patent No. 2587398 discloses a technique where an optical compensatory sheet obtained by coating, aligning and fixing a discotic liquid crystal on a triacetyl cellulose film is applied to a TN-mode liquid crystal cell to enlarge the viewing angle.
However, the demand with respect to the viewing angle dependency of a liquid crystal display device for televisions having a large screen and being envisaged for viewing from various angles is severe and this requirement cannot be satisfied by the above-described technique. Therefore, liquid crystal display devices different from the TN mode are being studied, such as IPS (in-plane switching) mode, OCB (optically compensatory bend) mode and VA (vertically aligned) mode. Particularly, the VA mode assures high contrast and relatively high production yield and is attracting attention as a liquid crystal display device for TV.
Incidentally, the cellulose acylate film is characterized in that the optical isotropy is high (the retardation value is low) as compared with other polymer films. Accordingly, a cellulose acylate film is commonly used for usage where optical isotropy is required, such as polarizing plate.
On the other hand, the optical compensatory sheet (retardation film) of liquid crystal display device is contrarily required to have optical anisotropy (high retardation value). Particularly, the optical compensatory sheet for VA is required to have a front retardation (e) of 30 to 200 nm and a retardation (Rth) of 70 to 400 nm in the film thickness direction. Therefore, a synthetic polymer film having a high retardation value, such as polycarbonate film and polysulfone film, is commonly used for the optical compensatory sheet. The front retardation value and the retardation value in the film thickness direction are optical characteristic values calculated according to the following formulae (2) and (3), respectively:Re=(nx−ny)×d  Formula (2)Rth={(nx+ny)/2−nz}×d  Formula (3)(wherein nx is a refractive index in the x direction in the film plane, ny is a refractive index in the y direction in the film plane, nz is a refractive index in the direction orthogonal to the film surface, and d is a thickness (μm) of film).
In this way, it is a general principle in the technical field of optical materials to use a synthetic polymer film when optical anisotropy (high retardation value) is required of the polymer film, and use a cellulose acylate film when optical isotropy (low retardation value) is required.
In EP-A-0911656, by reversing this conventional general principle, use of a cellulose acetate film with a high retardation value usable also for usage requiring optical anisotropy has been proposed. According to this technique, in order to realize a high retardation value with cellulose triacetate, an aromatic compound having at least two aromatic rings, particularly, a compound having 1,3,5-triazine ring is added and then the film is stretched. In general, the cellulose triacetate is a polymer material difficult to stretch and known to be hardly increased in the birefringence, but the additive is simultaneously oriented at the stretching, whereby the birefringence is elevated and a high retardation value is realized. This film can serve also as a protective film for the polarizing plate and an inexpensive thin-film liquid crystal display device can be advantageously provided.
JP-A-2002-71957 discloses an optical film comprising a cellulose ester which contains an acyl group having from 2 to 4 carbon atoms as a substituent and assuming that the substitution degree of acetyl group is A and the substitution degree of propionyl or butyryl group is B, satisfies both of the following conditions:2.0≦A+B≦3.0, andA<2.4,wherein the refractive index Nx in the slow axis direction and the refractive index Ny in the fast axis direction at a wavelength of 590 nm satisfy the following condition:0.0005≦Nx−Ny≦0.0050.
JP-A-2003-270442 discloses a polarizing plate for use in VA-mode liquid crystal display devices, which comprises a polarizer and an optically biaxial mixed fatty acid cellulose ester film and in which the optically biaxial mixed fatty acid cellulose ester film is disposed between the liquid crystal cell and the polarizer.